2011/10/12 Frodo Chao <frodogreat@gmail.com>

Hi,

I came upon this when playing with foldr and filter. Compare the two definitions:

testr n = foldr (\x y -> x && y) True [t | (_, t) <- zip [1 .. n] [True, True ..]]
testr' n = foldr (\x y -> y && x) True [t | (_, t) <- zip [1 .. n] [True, True ..]]

I tried these functions on ghci (The Glorious Glasgow Haskell Compilation System, version 7.0.3), and get the following results (with :set +s):

testr 1000000 => True
(0.01 secs, 7920832 bytes)
testr' 1000000 => True
(8.72 secs, 446585344 bytes)

This bizarre (at least to me) behavior also applies to ||. Should I mind the orderings of the parameters (especially the accumulator) in the function passed to foldr?

Thak you for reading.

Sincerely yours,
Frodo Chao